/last servlet

http://zwizwa.be/last/<file> is a redirect point for cached
dynamically generated files, such ad .pdf files from .tex documents,
and tarballs from the darcs source projects.
the main goal is to have a fixed entry point (service) that provides
these documents.
ideally, this would be demand-driven, but probably best to just use
cron jobs now.
next action: figure out how to redirect in scheme.
http://pre.plt-scheme.org/docs/html/web-server/response-structs_ss.html